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    <meta name="description" content="一个简单的重试机制只需要包括如下几步：  被调用的方法需要抛出异常 被调用方法需要放到一个死循环中，并捕获其异常 重试逻辑全部放到catch块中，如果不可以重复执行则在catch块内直接抛出异常即可，否则吞掉被调用方法抛出的异常，继续循环。简单一句：不可重复执行就在catch块内抛出异常!">
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            <h1 id="seo-header">怎么实现一个简单的重试机制</h1>
            
            
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                <p>一个简单的重试机制只需要包括如下几步：</p>
<ol>
<li>被调用的方法需要抛出异常</li>
<li>被调用方法需要放到一个死循环中，并捕获其异常</li>
<li>重试逻辑全部放到catch块中，如果不可以重复执行则在catch块内直接抛出异常即可，否则吞掉被调用方法抛出的异常，继续循环。简单一句：不可重复执行就在catch块内抛出异常!<span id="more"></span></li>
</ol>
<p>下面给一个简单retry的基本框架代码，入参和返回可以自行定义，这里只做示例：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><code class="hljs java"><span class="hljs-keyword">public</span> ReturnStructure <span class="hljs-title function_">executeFunctionName</span><span class="hljs-params">(<span class="hljs-type">int</span> execCount, ExecContext context)</span> &#123;  <br>    <span class="hljs-comment">// execCount: 当前的重试次数  </span><br>    <span class="hljs-comment">// context: 重试的上下文，可以用来存储状态等信息，一般这个是需要输入给被调用方法的，以便获取到执行过程中的一些信息以便在重试逻辑里做更多的判断。如果不需要，可以不传。</span><br>    <span class="hljs-comment">//1.死循环</span><br>    <span class="hljs-keyword">for</span> (<span class="hljs-type">int</span> <span class="hljs-variable">execCount</span> <span class="hljs-operator">=</span> <span class="hljs-number">1</span>;; execCount++) &#123;<br>        <span class="hljs-keyword">try</span> &#123;<br>            <span class="hljs-comment">// 执行被调用方法  </span><br>            <span class="hljs-keyword">return</span> needRetryFunctionName(context);<br>        &#125; <span class="hljs-keyword">catch</span> (Exception e) &#123;<br>            <span class="hljs-comment">// 捕获被调用方法抛出的异常，如果判断重复比较简单也可以不需要单独一个shouldRetry方法，直接在catch块内判断。</span><br>            <span class="hljs-keyword">if</span> (shouldRetry(e, execCount, context)) &#123;<br>                <span class="hljs-comment">// 重试,也可以添加其他判断逻辑如果不满足，也可以抛出异常终止循环。</span><br>                <span class="hljs-keyword">continue</span>; <span class="hljs-comment">//这句可有可无</span><br>            &#125; <span class="hljs-keyword">else</span> &#123;<br>                <span class="hljs-comment">// 抛出异常，中断循环，即中断重试。</span><br>                <span class="hljs-keyword">throw</span> e;<br>            &#125;<br>        &#125;<br>    &#125;<br>&#125;<br></code></pre></td></tr></table></figure>

<p>下面是一个简单的重试机制的例子：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br></pre></td><td class="code"><pre><code class="hljs java"><span class="hljs-keyword">public</span> CloseableHttpResponse <span class="hljs-title function_">execute</span><span class="hljs-params">(</span><br><span class="hljs-params">        <span class="hljs-keyword">final</span> HttpRoute route,</span><br><span class="hljs-params">        <span class="hljs-keyword">final</span> HttpRequestWrapper request,</span><br><span class="hljs-params">        <span class="hljs-keyword">final</span> HttpClientContext context,</span><br><span class="hljs-params">        <span class="hljs-keyword">final</span> HttpExecutionAware execAware)</span> <span class="hljs-keyword">throws</span> IOException, HttpException &#123;<br>    Args.notNull(route, <span class="hljs-string">&quot;HTTP route&quot;</span>);<br>    Args.notNull(request, <span class="hljs-string">&quot;HTTP request&quot;</span>);<br>    Args.notNull(context, <span class="hljs-string">&quot;HTTP context&quot;</span>);<br>    <span class="hljs-keyword">final</span> Header[] origheaders = request.getAllHeaders();<br>    <span class="hljs-comment">//这个for循环记录了当前http请求的执行次数,如果执行期间没有异常抛出实际就是一个死循环</span><br>    <span class="hljs-keyword">for</span> (<span class="hljs-type">int</span> <span class="hljs-variable">execCount</span> <span class="hljs-operator">=</span> <span class="hljs-number">1</span>;; execCount++) &#123;<br>        <span class="hljs-keyword">try</span> &#123;<br>            <span class="hljs-comment">//调用基础executor执行http请求,execAware用来存储执行过程的状态或关键数据供异常处理过程中逻辑使用</span><br>            <span class="hljs-keyword">return</span> <span class="hljs-built_in">this</span>.requestExecutor.execute(route, request, context, execAware);<br>        &#125; <br>        <span class="hljs-comment">// 核心逻辑全在异常处理里面，判断是否需要中断，是否需要继续执行下一次for循环。如果需要中断通过抛出异常中断！！！否则会继续执行下一次for循环</span><br>        <span class="hljs-keyword">catch</span> (<span class="hljs-keyword">final</span> IOException ex) &#123;<br>            <span class="hljs-comment">//发生IO异常的时候,判断上下文是否已经中断，如果中断则抛异常退出</span><br>            <span class="hljs-keyword">if</span> (execAware != <span class="hljs-literal">null</span> &amp;&amp; execAware.isAborted()) &#123;<br>                <span class="hljs-built_in">this</span>.log.debug(<span class="hljs-string">&quot;Request has been aborted&quot;</span>);<br>                <span class="hljs-keyword">throw</span> ex;<br>            &#125;<br>            <span class="hljs-comment">//根据重试策略，判断当前执行状况是否要重试，如果是则进入下面逻辑，retryRequest的逻辑框架见后文。</span><br>            <span class="hljs-keyword">if</span> (retryHandler.retryRequest(ex, execCount, context)) &#123;<br>                <span class="hljs-keyword">if</span> (<span class="hljs-built_in">this</span>.log.isInfoEnabled()) &#123;<br>                    <span class="hljs-built_in">this</span>.log.info(<span class="hljs-string">&quot;I/O exception (&quot;</span>+ ex.getClass().getName() +<br>                            <span class="hljs-string">&quot;) caught when processing request to &quot;</span><br>                            + route +<br>                            <span class="hljs-string">&quot;: &quot;</span><br>                            + ex.getMessage());<br>                &#125;<br>                <span class="hljs-keyword">if</span> (<span class="hljs-built_in">this</span>.log.isDebugEnabled()) &#123;<br>                    <span class="hljs-built_in">this</span>.log.debug(ex.getMessage(), ex);<br>                &#125;<br>                <span class="hljs-comment">//判断当前请求是否可以被重复发起</span><br>                <span class="hljs-keyword">if</span> (!RequestEntityProxy.isRepeatable(request)) &#123;<br>                    <span class="hljs-built_in">this</span>.log.debug(<span class="hljs-string">&quot;Cannot retry non-repeatable request&quot;</span>);<br>                    <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">NonRepeatableRequestException</span>(<span class="hljs-string">&quot;Cannot retry request &quot;</span> +<br>                            <span class="hljs-string">&quot;with a non-repeatable request entity&quot;</span>, ex);<br>                &#125;<br>                <span class="hljs-comment">//更新requset的HEADER</span><br>                request.setHeaders(origheaders);<br>                <span class="hljs-keyword">if</span> (<span class="hljs-built_in">this</span>.log.isInfoEnabled()) &#123;<br>                    <span class="hljs-built_in">this</span>.log.info(<span class="hljs-string">&quot;Retrying request to &quot;</span> + route);<br>                &#125;<br>            &#125;<br>            <span class="hljs-comment">//如果重试策略判断不能重试了，则根据异常状态抛异常，退出当前流程 </span><br>            <span class="hljs-keyword">else</span> &#123;<br>                <span class="hljs-keyword">if</span> (ex <span class="hljs-keyword">instanceof</span> NoHttpResponseException) &#123;<br>                    <span class="hljs-keyword">final</span> <span class="hljs-type">NoHttpResponseException</span> <span class="hljs-variable">updatedex</span> <span class="hljs-operator">=</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">NoHttpResponseException</span>(<br>                            route.getTargetHost().toHostString() + <span class="hljs-string">&quot; failed to respond&quot;</span>);<br>                    updatedex.setStackTrace(ex.getStackTrace());<br>                    <span class="hljs-keyword">throw</span> updatedex;<br>                &#125;<br>                <span class="hljs-keyword">throw</span> ex;<br>            &#125;<br>        &#125;<br>    &#125;<br>&#125;<br></code></pre></td></tr></table></figure>

<p>Retry的详细判断逻辑框架如下,如果只是简单的判断，可以直接写在上面的主逻辑中，并不需要单独出一个判断函数：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><code class="hljs java"><span class="hljs-keyword">public</span> <span class="hljs-type">boolean</span> <span class="hljs-title function_">shouldRetry</span><span class="hljs-params">(Throwable ex, <span class="hljs-type">int</span> execCount, RetryContext context)</span> &#123;  <br>    <span class="hljs-comment">// 你的重试逻辑在这里  </span><br>    <span class="hljs-comment">// ex: 抛出的异常  </span><br>    <span class="hljs-comment">// execCount: 当前的重试次数  </span><br>    <span class="hljs-comment">// context: 重试的上下文，可以用来存储状态等信息  </span><br>    <span class="hljs-keyword">return</span> shouldRetryBoolValue; <span class="hljs-comment">// 返回一个布尔值表示是否需要重试  </span><br>&#125;<br></code></pre></td></tr></table></figure>

<p>上面示例retryRequest的源代码如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><code class="hljs java"><span class="hljs-keyword">public</span> <span class="hljs-type">boolean</span> <span class="hljs-title function_">retryRequest</span><span class="hljs-params">(</span><br><span class="hljs-params">        <span class="hljs-keyword">final</span> IOException exception,</span><br><span class="hljs-params">        <span class="hljs-keyword">final</span> <span class="hljs-type">int</span> executionCount,</span><br><span class="hljs-params">        <span class="hljs-keyword">final</span> HttpContext context)</span> &#123;<br>    Args.notNull(exception, <span class="hljs-string">&quot;Exception parameter&quot;</span>);<br>    Args.notNull(context, <span class="hljs-string">&quot;HTTP context&quot;</span>);<br>    <span class="hljs-keyword">if</span> (executionCount &gt; <span class="hljs-built_in">this</span>.retryCount) &#123;<br>        <span class="hljs-comment">// Do not retry if over max retry count</span><br>        <span class="hljs-keyword">return</span> <span class="hljs-literal">false</span>;<br>    &#125;<br>    <span class="hljs-keyword">if</span> (<span class="hljs-built_in">this</span>.nonRetriableClasses.contains(exception.getClass())) &#123;<br>        <span class="hljs-keyword">return</span> <span class="hljs-literal">false</span>;<br>    &#125;<br>    <span class="hljs-keyword">for</span> (<span class="hljs-keyword">final</span> Class&lt;? <span class="hljs-keyword">extends</span> <span class="hljs-title class_">IOException</span>&gt; rejectException : <span class="hljs-built_in">this</span>.nonRetriableClasses) &#123;<br>        <span class="hljs-keyword">if</span> (rejectException.isInstance(exception)) &#123;<br>            <span class="hljs-keyword">return</span> <span class="hljs-literal">false</span>;<br>        &#125;<br>    &#125;<br>    <span class="hljs-keyword">final</span> <span class="hljs-type">HttpClientContext</span> <span class="hljs-variable">clientContext</span> <span class="hljs-operator">=</span> HttpClientContext.adapt(context);<br>    <span class="hljs-keyword">final</span> <span class="hljs-type">HttpRequest</span> <span class="hljs-variable">request</span> <span class="hljs-operator">=</span> clientContext.getRequest();<br><br>    <span class="hljs-keyword">if</span>(requestIsAborted(request))&#123;<br>        <span class="hljs-keyword">return</span> <span class="hljs-literal">false</span>;<br>    &#125;<br><br>    <span class="hljs-keyword">if</span> (handleAsIdempotent(request)) &#123;<br>        <span class="hljs-comment">// Retry if the request is considered idempotent</span><br>        <span class="hljs-keyword">return</span> <span class="hljs-literal">true</span>;<br>    &#125;<br><br>    <span class="hljs-keyword">if</span> (!clientContext.isRequestSent() || <span class="hljs-built_in">this</span>.requestSentRetryEnabled) &#123;<br>        <span class="hljs-comment">// Retry if the request has not been sent fully or</span><br>        <span class="hljs-comment">// if it&#x27;s OK to retry methods that have been sent</span><br>        <span class="hljs-keyword">return</span> <span class="hljs-literal">true</span>;<br>    &#125;<br>    <span class="hljs-comment">// otherwise do not retry</span><br>    <span class="hljs-keyword">return</span> <span class="hljs-literal">false</span>;<br>&#125;<br></code></pre></td></tr></table></figure>


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